This invention relates to a thermal infrared detector having a thermal isolation structure and, in particular, to a thermal infrared detector capable of avoiding occurrence of image blur even if operation is carried out at a high frame rate and of suppressing increase in 1/f noise and to a method of producing the same.
For example, a conventional thermal infrared detector of the type is disclosed in Japanese Unexamined Patent Publication JP 2001-215151 A. The thermal infrared detector has a shield above a photosensitive portion.
Referring to FIG. 1, the conventional thermal infrared detector will be described.
The thermal infrared detector comprises a substrate 1, a plurality of diaphragms 5p each of which forms a diaphragm structure spatially separated from the substrate 1 and serves as an infrared photosensitive portion, a plurality of beams 6p supporting the diaphragms 5p, and a plurality of shields 12p each of which extends outward from an outer perimeter of the diaphragm 5p. The thermal infrared detector comprises a substrate 1 and a plurality of infrared photosensitive portions each of which forms a diaphragm structure spatially separated from the substrate 1. Each infrared photosensitive portion comprises a diaphragm 5p, a plurality of beams 6p supporting the diaphragm 5p, and a plurality of shields 12p which extends outward from an outer perimeter of the diaphragm 5p. On the substrate 1 , an infrared reflecting film 2 and a first dielectric protective film 3 are formed. Each of the diaphragms 5p comprises a thermistor-bolometer thin film (hereinafter briefly be called a bolometer thin film) 7p, a pair of electrode portions 10p, and a second dielectric protective film 14a surrounding the bolometer thin film 7p and the electrode portions l0p. The shield 12p extending outward from the outer perimeter of the diaphragm 5p covers the beam 6p with an interposed space. The beam 6p comprises a metal wiring 9p and a third dielectric protective film 14b surrounding the metal wiring 9p. 
Infrared radiation or light incident to the diaphragm 5p on its upper surface is partly received and absorbed by a material of the diaphragm 5p and partly passes through the diaphragm 5p. The infrared light which has passed through the diaphragm 5p is reflected by the infrared reflecting film 2 on the substrate 1 and is again incident to the diaphragm 5p to be absorbed. The infrared light absorbed as mentioned above heats the diaphragm 5p as the photosensitive portion to change the resistance of the bolometer thin film 7p. By applying a bias electric current to the bolometer thin film 7p, the amount of infrared light received by the diaphragm 5p is converted into voltage change which is read as a signal.
The technique disclosed in JP 2001-215151 A is applicable, for example, to a thermal infrared array sensor in which the number of pixels is 320×240 and the pixel pitch is 40 μm. In the thermal infrared array sensor, the bolometer thin film 7p comprises a vanadium oxide film having a thickness of 100 nm and a temperature coefficient of resistance of −2%/K. The dielectric protective film 14a of the diaphragm 5p comprises a silicon nitride film having a thickness of 500 nm. The electrode portion 10p on the diaphragm 5p comprises a titanium film having a thickness of 100 nm. The dielectric protective film 14b of the beam 6p comprises a silicon nitride film having a thickness of 500 nm, a width of 2.6 μm, and a length of 86 μm. The metal wiring 9p of the beam 6p comprises a titanium film having a thickness of 100 nm, a width of 1 μm, and a length of 86 μm. A dielectric protective film of the shield 12p comprises a silicon nitride film having a thickness of 500 nm. The shield 12p has an aperture ratio of 90%. The number of bends of the beam is two or slightly more. In the thermal infrared array sensor of a thermal isolation structure having the above-mentioned specification, the thermal capacity is 1.4×10−9 J/K, the thermal conductance is 8.2×10−2 W/K, and the thermal time constant is 15 ms. For a F/1 optical system, an infrared image having a temperature resolution of about 30 mK_could be obtained.
Japanese Unexamined Patent Publication JP 2001-153720 A discloses another thermal infrared detector having a thermal isolation structure. In this publication, the number of bends of the beam which gives a best temperature resolution is represented by a beam length index which is a function of a pixel size of the infrared detector.
Referring to FIG. 2, the thermal infrared detector will be described. A pixel 31 comprises a diaphragm 32 as a photosensitive portion, a pair of beams 33a and 33b supporting the diaphragm 32, and a pair of cell contact electrodes 35a and 35b. The beams 33a and 33b are provided with wirings 36a and 36b, respectively. The diaphragm 32 comprises a bolometer thin film, electrode portions, and a dielectric protective film surrounding the thin film and the electrode portions. The diaphragm 32 is spatially separated from the beams 33a and 33b by slits 34a and 34b, respectively. The diaphragm 32 is spatially separated from a lower substrate (not shown) to be afloat in the air, like in FIG. 1.
The technique disclosed in JP 2001-153720 A is applicable, for example, to a thermal infrared array sensor in which the number of pixels is 320×240 and the pixel pitch is 25 μm. In the array sensor, the bolometer thin film comprises a vanadium oxide film having a thickness of 100 nm and a temperature coefficient of resistance of −2%/K. The dielectric protective film of the diaphragm 32 comprises a silicon nitride film having a thickness of 500 nm. The electrode portion of the diaphragm 32 comprises a titanium alloy film having a thickness of 100 nm. A dielectric protective film of each of the beams 33a and 33b comprises a silicon nitride film having a thickness of 500 nm, a width of 2 μm, and a length of 32 μm. Each of the metal wirings 36a and 36b of the beams 33a and 33b comprises a titanium alloy film having a thickness of 100 nm, a width of 1 μm, and a length of 32 μm. The diaphragm 32 as an infrared photosensitive portion has an aperture ratio of 51%. Each of the slits 34a and 34b has a width of 0.5 μm. In this case, an optimum value of the number of bends of the beam is 1.3. In the thermal infrared array sensor of a thermal isolation structure having the above-mentioned specification, the thermal capacity is 3.3×10−10 J/K, the thermal conductance is 8.5×10−2 W/K, and the thermal time constant is 3.9 ms. For a F/1 optical system, an infrared image having a temperature resolution of 100 mK could be obtained.
By combining the technique disclosed in JP 2001-215151 A and JP 2001-153720 A, it is possible to propose a high-sensitivity thermal infrared detector which has the number of bends of a beam or the beam length index giving a best temperature resolution, which has a shield extending outward from an outer perimeter of the diaphragm, and which has a thermal isolation structure.
As a specific example, description will be made of the case where the thermal infrared detector described in conjunction with JP 2001-153720 A is provided with a shield of an aperture ratio of 92%. As a dielectric protective film of the shield, a silicon nitride film having a thickness of 500 nm was used. As a result, the thermal capacity of 5.2×10−10 J/K, the thermal conductance of 8.8×10−2 W/K, and the thermal time constant of 6 ms were obtained. For a F/1 optical system, an infrared image having a temperature resolution of 60 mK could be obtained.
Furthermore, an uncooled infrared sensor is disclosed in U.S. Pat. No. 6,046,485. Referring to FIG. 3A, the uncooled infrared sensor having a thermal isolation structure has a pixel 60. The pixel 60 comprises a photosensitive portion including an infrared absorbing portion 65 and a pair of outer bolometer portions 56 and 58, a contact portion including a pair of cell contact electrodes 42 and 44, and a pair of beams 46 and 48 electrically and mechanically connecting the photosensitive portion and the contact portion. The photosensitive portion and the beams 46 and 48 are connected via junctions 52 and 54. As seen from FIG. 3B, the photosensitive portion (56, 58, 65) is supported by the beams 46 and 48 to be afloat above the substrate. Thus, the thermal isolation structure is formed.
In the above-mentioned United States patent, the photosensitive portion has a structure in which the infrared absorbing portion 65 alone is reduced in thickness without reducing the thickness of a protective film 64 surrounding the outer bolometer portions 56 and 58. With the above-mentioned structure, the thermal capacity of the photosensitive portion (56, 58, 65) is decreased and the thermal time constant is reduced. Thus, the uncooled infrared sensor having a thermal isolation structure and operable at a high frame rate is achieved.
However, the conventional thermal infrared detectors mentioned above are disadvantageous in the following respects.
At first, consideration will be made of the case where the techniques of JP 2001-215151 A and JP 2001-153720A are combined. In this case, if the temperature resolution is improved, the thermal time constant is increased. When the infrared detector is operated at a high frame rate of 120 Hz, image blur occurs. This is because, in the above-mentioned thermal infrared detector, the beam and the diaphragm are equal in thickness to each other.
The structure described in U.S. Pat. No. 6,046,485 is disadvantageous in that 1/f noise from the bolometer portion is large. This is presumably because a bolometer material is present only at an outer perimeter of the photosensitive portion so that the total number of carriers is small and, therefore, the 1/f noise is large. The above presumption is based on the theory that the 1/f noise of the bolometer is smaller as the total number of carriers in the bolometer material is greater (see P. H. Handel, Phys, Rev. A, Vol. 22, 1980, p. 745).